1. Field of the Invention
The present invention relates to a variable attenuator, and more particularly, to a variable attenuator for variously controlling the amplitude (gain) of an analog signal by a digital signal.
2. Description of the Related Art
Recently, it has been required that audio or video devices, or other electronic devices, are able to control the amplitude (level) of an analog signal, e.g., a sound signal, by digital equipment such a microcomputer, remote controller, and the like, without using a hand-operated volume control.
A conventional digitally controlled variable attenuator (variable signal attenuator) digitally controls an analog signal attenuation by switching one of a plurality of switches. In the conventional variable attenuator, the output impedance changes in accordance with the switching state of the switches. Further, when connecting a load resistor having a specific resistance value to the variable attenuator, the attenuation value of each node connected to the switches is independently changed, and thereby the ratios of those attenuation values are also changed and the values of the attenuation resistors must be calculated again.
Further, when applying the variable attenuator to a variable attenuator system having two input signals and using an operational amplifier, a signal output from a first signal source is controlled by the variable attenuation portion and supplied to an inverted input terminal of the operational amplifier, and a signal output from a second signal source is set by level setting resistors and supplied to a normal input terminal of the operational amplifier. When switching the switches so as to obtain an intermediate output voltage by controlling a gain of the first input voltage, there is a problem that a gain defined from a second input voltage to the output voltage is changed. Further, in the conventional variable attenuator, the number of the required attenuation resistors and switches increases in accordance with an increase in the number of bits in the control signal.